1. Field
The instant invention relates to elastomeric-covered shrouded impellers for centrifugal pumps.
2. Prior Art
Shrouded impellers in which the vanes and vane passages are enclosed between a pair of opposed shrouds or disks are relatively commonplace in centrifugal pumps. Shrouded impellers are available in both metal and elastomer-covered metal constructions. Metal impellers are typically utilized in non-abrasive, non-corrosive environments. Elastomer-covered impellers, because of expense and difficulty in making same, are typically utilized only where abrasive or corrosive resistance is required, for example, in slurry pumps handling abrasive or gritty solids in a liquid media or in dealing with corrosive liquids such as acids and the like.
An elastomeric-covered impeller is formed about a metal insert. The technique involves placing the metal insert within a mold and providing core elements which provide for the voids within the impeller after molding. Elastomeric material is forced generally under pressure into the molds so that those spaces which exist between the metal insert and the core elements are filled with rubber thereby forming the elastomeric-covered shrouded impeller.
A typical rubber-covered impeller is shown in FIG. 1 in an elevational view showing the peripheral edge of the impeller with vane passage openings shown at the periphery. The formation of the throat opening and vane passages in the molding process is relatively straight-forward in this type of construction.
An elevational view of the vanes of a shrouded impeller along section lines 2--2 of FIG. 1, is illustrated in FIG. 2. The spacing between adjacent vanes is closer near the center of the impeller than around the outer edges of the vanes. In the orientation of the impeller illustrated in FIG. 2, the rotation of the impeller is counter-clockwise.
In the arrangement illustrated in FIG. 3, the vane core, which is a portion of the mold which forms the vane passage, has a uniform width "w" between the inner walls of the front and rear shroud. For the purposes of description herein of shrouded impellers, the front shroud is the shroud containing the inlet opening in the throat of the impeller. Thus, the vane core may be easily extracted by a force perpendicular to the central axis of the impeller.
A slight variation to the arrangement illustrated in FIG. 3 is that illustrated in FIG. 4 which is another prior art arrangement. The illustration of FIG. 4 shows some curvature of the inner walls of the front and rear shrouds. This wall curvature is to provide a flow channel from the inlet throat of the impeller into the vane passage which provides a gradual change of direction to accomplish the 90.degree. change of direction from axial inlet flow to radial outlet flow. The vane passage of the impeller of FIG. 4 is formed by a pair of vane core members, A and B, whereby, the width W.sub.A and width W.sub.B of each core member is smaller than the width "b" of the peripheral vane passage width. Extraction of these core members is perpendicular to the central axis of the impeller and is in the order of core "A" being first removed and then core "B" being later removed.
In metal impellers with enclosed vanes the formation of shrouds with curved inner walls has been practiced for quite some time. Metal impellers are generally formed by sand casting, whereby, the formation of curved interior walls of the forward and rear shrouds has been easily achieved since solid core members are not used in the casting process. Thus, the achievement of a channel connecting the inlet throat with the vane passage in a manner such that the channel encounters no sharp angle restrictions has been long practiced with metal impellers.
The presence of a right-angle corner such as that present in the construction illustrated in FIG. 3, may cause velocity loss as well as turbulence near the square corner and cause erosion of the elastomeric covering on the back shroud in the area directly opposite to the square corner on the front shroud.